Metabolic studies with whole body M3 mAChR KO mice showed that the lack of M3 receptors is consistently associated with reduced food intake and increased energy expenditure (Gautam et al., Cell Metab. 4, 363-375, 2006). We demonstrated that this increase in energy expenditure is due to an elevated rate in basal and total oxygen consumption, associated with increased locomotor activity, body temperature, and sympathetic tone. Moreover, the whole body M3 receptor KO mice were protected against different forms of experimentally- and genetically-induced obesity and obesity-associated disorders.[unreadable] The M3 receptor is found in many peripheral tissues, including glands and smooth muscle tissues, and most areas of the CNS. Therefore, to gain insight into the mechanisms underlying the striking metabolic phenotypes displayed by the whole body M3 receptor KO mice, we started to use Cre/LoxP technology to generate mutant mice that lack M3 receptors only in specific tissues or cell types. [unreadable] The liver plays a key role in maintaining normal glucose and energy homeostasis. We recently demonstrated that the M3 receptor subtype is the only mAChR expressed by the mouse liver (hepatocytes). To assess the physiological role of these liver M3 receptors in regulating glucose and energy homeostasis in vivo, we employed Cre/loxP technology to generate mutant mice lacking M3 receptors in hepatocytes only. Moreover, to study the metabolic effects of enhanced signaling through liver M3 receptors, we also generated mutant mice selectively overexpressing this receptor subtype in hepatocytes. The phenotypic analysis of these animals is currently ongoing.[unreadable] Energy homeostasis and food intake are known to be under the strict control of regulatory centers in the hypothalamus. Among the many cell types found in the hypothalamus, the POMC- and AGRP-containing neurons are known to play key roles in regulating appetite and energy homeostasis. We demonstrated that essentially all POMC- and AGRP-containing neurons express M3 mAChRs. We therefore started to use Cre/loxP technology to generate mutant mice lacking M3 receptors in POMC- and AGRP-containing neurons only. Phenotypic analysis of these mutant animals should shed new light on the roles of hypothalamic M3 receptors in the regulation of energy homeostasis and food intake.[unreadable] It is likely that these studies will lead to the identification of novel pharmacologic targets for the treatment of obesity and associated metabolic disorders.